Only the latest models do sadly.
The D5200, D5300, D7100, D800 does support bulb over USB AFAIK. I'm assuming all cameras built after these models will.
The D5100 and D7000 do not. However, they have better sensors for astronomy. So it all depends what your goal is.
I believe none the D## (2 digits) model support bulb over USB.
Hope this helps,
Can you elaborate on why the sensors in the 5100/7000 are better?